GMC Troubleshooting Questions? Ask a Mechanic for Answers
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I would not think you would have to go to a 4.10 rear end.
When you say its chugging at 70km. Is the trans lockup converter engaged?
Do you have a programmer installed that lets you change shift points?
The problem of chugging is not the rear end. Unless you want more performance at the low end than you need to change to a 4.10 rear end.
The 3.73 most I would go.
I think your trans is up shifting to soon.